In the manufacture of semiconductor devices, semiconductor wafers, such as silicon semiconductor wafers, are subjected to a number of processing steps. One such process step includes the depositing or forming of layers on the silicon semiconductor wafer. One such process used to form the layers is known as chemical vapor deposition (CVD), wherein reactive gases are introduced into a chamber, e.g., a CVD process tool, containing the semiconductor wafers. The reactive gases facilitate a chemical reaction that causes a layer to form on the semiconductor wafers. One exemplary deposition process is the formation of polysilicon by reacting nitrogen (N.sub.2) and silane (SiH.sub.4) in a furnace. Various types of CVD process tools may be used. For example, the CVD process tool may be an atmospheric-pressure (AP) CVD process tool, a low-pressure (LP) CVD process tool, or a plasma-enhanced CVD process tool.
There are many factors that affect the deposition rate of a deposition tool. These factors include, among other things, the flow rate of reactive gases through the chamber and the temperature of the chamber. Typically, to determine the deposition rate for a particular tool (e.g., when it is first placed in service or after a maintenance event), a series of qualification semiconductor wafers are processed and the resultant thickness of the process layer is measured. The measurements are used to estimate the deposition rate of the tool. Deposition times for subsequently processed semiconductor wafers are determined based on the anticipated deposition rate. Normal variations in temperature and reactant flow rate may cause a deviation in the deposition rate from the anticipated rate, causing the process to exceed a control limit.
The CVD tool deposits a film (layer) onto a semiconductor wafer. Typically, when the semiconductor wafer leaves the CVD tool, the film exceeds the desired target final thickness. To reduce the film to the desired target thickness, the semiconductor wafer is then fed to a CMP (Chemical Mechanical Planarization) process tool. This process tool “polishes” the film to remove a portion of the film, making it thinner, so that the film is the desired thickness when it exits the CMP process tool.
For the purposes of this disclosure, the thickness of the film on a semiconductor wafer when it is input to the CMP process tool is referred to as the “input thickness.” The desired thickness of the film when it exits the CMP process tool is referred to as the “desired thickness.” The actual thickness of the film when it exits the CMP process tool is referred to as the “final thickness.” An ideal goal of the semiconductor manufacturing process is that the final thickness is equal to the desired thickness. In a practical sense, the final thickness should be within a predefined tolerance of the desired thickness.
It is desirable to have extremely repeatable manufacturing processes for semiconductor devices. Therefore it is necessary to minimize the variation in the thickness of the film during the manufacturing process. E.g., a batch of semiconductor wafers made two months ago should ideally be identical to a batch of semiconductor wafers made yesterday.
In an effort to produce consistent film thickness on each semiconductor wafer, the input thickness is periodically measured on a sampling of semiconductor wafers after they leave the CVD process tool. This generates a “running average” thickness. This average thickness data is fed to the CMP process tool. The CMP process tool, assuming this average thickness to be the input thickness of the film on a particular semiconductor wafer, can calculate the appropriate polish time, thereby attempting to achieve the desired thickness when the semiconductor wafer exits the CMP process tool.
The above method does not yield optimal results. This is because semiconductor wafers are only periodically sampled as they leave the CVD process tool to monitor input thickness. There is a desire to minimize the number of wafers sampled because measurement is considered to be a manufacturing overhead, and causes decreased manufacturing throughput. However, some measurement is necessary because there can be considerable variation in the thickness of the film as it leaves the CVD process tool. A prime reason for this variation is that the CVD process tool typically has multiple chambers. These chambers may have slightly different operating temperatures, which causes variations in semiconductor wafers, depending on what chamber they were in when the film was deposited.
Referring now to FIG. 1, a prior art Film Deposition and Polish process system 100 is shown. The system comprises at least one CVD (Chemical Vapor Deposition) Process Tool 104, a CMP Process Tool 106, a Measurement Tool 110, a Data Management System 112, and a CMP R2R (Chemical Mechanical Planarization Run-to-Run) Controller 114. In the block diagrams of this disclosure, the flow of semiconductor wafers is indicated by the wide arrows denoted W1, W2, W3, and W4. Thin solid arrows (122,124,126, and 128) indicate data communication links between the various devices.
CVD Process Tool 104 uses Chemical Vapor Deposition techniques that are well known in the industry to apply a film to a surface of a semiconductor wafer. A majority of semiconductor wafers (not shown) leave the CVD Process Tool 104, and proceed directly to the CMP Process Tool 106 via path W1. The thickness of the film on the semiconductor wafers when they are input to the CMP process tool 106 is referred to as the input thickness. The CMP Process Tool 106 removes a portion of the film that was applied with the CVD Process Tool 104 to achieve a final thickness. In order to compensate for variations in input thickness, a portion of the semiconductor wafers that exit the CVD Process Tool 104 travel via path W2 to a Measurement Tool 110. The Measurement Tool 110 measures the input thickness of the film on a sampling of semiconductor wafers. As mentioned previously, due to throughput considerations, it is usually not practical to send all semiconductor wafers via path W2 to the Measurement Tool 110. Only a portion of the semiconductor wafers are sent. For example, every fourth semiconductor wafer might be sent to Measurement Tool 110, with all other semiconductor wafers proceeding directly to CMP Process Tool 106. After a semiconductor wafer leaves Measurement Tool 110, it proceeds via path W3 back to the manufacturing path to enter the CMP Process Tool 106. When the CMP Process is complete, semiconductor wafers exit the CMP Process Tool 106 via path W4. The goal is to have the final thickness of the film of the semiconductor wafers exiting the CMP Process Tool 106 at the desired thickness.
The Measurement Tool 110 sends the measured film input thickness data to a Data Management System 112 via data communication link 122. Data Management System 112 may be comprised of a computer having data acquisition means, network communication means, and data storage means, as is well known in the industry. The Data Management System 112 may also have a general purpose microprocessor that executes software to analyze the data supplied via Measurement Tool 110. In one case, the data analysis may comprise calculating a running average of film input thickness. It is possible that the Data Management System 112 is integrated with the CMP R2R Controller 114.
The CMP R2R Controller 114 is a factory automation controller that receives input from Data Management System 112 via data communication link 124. CMP Process Tool 106 preferably has an Integrated Measurement system 108 that measures the final thickness of the film on a sample of the semiconductor wafers that have undergone the CMP process. This information is also fed to Data Management System 112 via data communication link 126. The final thickness measurements are used to monitor the CMP process to ensure it is within acceptable limits. The Data Management System 112 supplies the final thickness to a CMP R2R Controller 114 via data communication link 124. The CMP R2R Controller uses the data received from the Data Management System 112 to compute appropriate parameter values for the remaining semiconductor wafers on the CMP Process Tool 106. The appropriate parameter values are sent from the CMP R2R Controller 114 to the CMP Process Tool 106 via data communication link 128, and override the existing parameter values that were specified in the tool process recipe. The tool process recipe contains a list of parameters with values for a specific process operation. The parameters may include, but are not limited to, the polishing time, polishing rate, down force, slurry particle size, pH of the slurry, back pressure, and slurry flow rate of the CMP Process Tool 106. Having an accurate film input thickness is essential for fine tuning the recipe of the CMP Process Tool 106.
The average film input thickness is an approximation that is intended to be representative of the input film thickness of a given semiconductor wafer that is entering the CMP Process Tool 106. However, the method for calculating and using average film input thickness of the system 100 shown in FIG. 1 has some shortcomings. It does not account for variations amongst different CVD Process Tools. For example, while only on CVD Process Tool 106 was shown, it is often desirable to use multiple CVD Process Tools in a high-volume manufacturing operation. Furthermore, within each CVD Process Tool, there are typically multiple semiconductor wafer processing chambers, providing for simultaneous processing of more than one semiconductor wafer. Each processing chamber within a particular CVD Process Tool has its own slight variation as compared with other processing chambers within a CVD Process Tool. The system 100 of FIG. 1 does not take this variation into account. Therefore, the running average film input thickness is not an optimal representation of the film input thickness of a particular semiconductor wafer. It is desirable to improve accuracy of the average film input thickness without requiring additional measurements that would slow down production and reduce throughput of the manufacturing line.
CMP R2R Controller 114 may use standard factory automation methods for semiconductor manufacturing, such as the Advanced Process Control (APC) Framework. CIM (SEMI E81-0699-Provisional Specification for CIM Framework Domain Architecture) and APC (SEMI E93-0999-Provisional Specification for CIM Framework Advanced Process Control Component) specifications are publicly available from SEMI (The Semiconductor Equipment and Materials Institute, commonly known as SEMI, is an organization headquartered in San Jose, Calif., that publishes various specifications for the semiconductor industry). The APC is a preferred platform from which to implement the control strategy taught by the present invention. In some embodiments, the APC can be a factory-wide software system; therefore, the control strategies taught by the present invention can be applied to virtually any of the semiconductor manufacturing tools on the factory floor. The APC framework also allows for remote access and monitoring of the process performance. Furthermore, by utilizing the APC framework, data storage can be more convenient, more flexible, and less expensive than local drives. The APC platform allows for more sophisticated types of control because it provides a significant amount of flexibility in writing the necessary software code. These methods are well known in the industry, and are disclosed in U.S. Pat. No. 6,708,129 (entitled “Method and apparatus for semiconductor wafer-to-semiconductor wafer control with partial measurement data”) which is herein incorporated by reference in its entirety.
For maximum process control, it would be ideal to measure the input thickness of the film on every semiconductor wafer being fed to the CMP process tool. However, this is not practical due to the time it takes to perform this measurement. Therefore, what is needed is an improved method that increases the accuracy of the film thickness information supplied to the CMP process tool, without causing excessive slowdown in the manufacturing throughput.